The invention is directed to a process for separating grain mixture, e.g. for separating heavy inclusions such as stones from grain product, in which the product is guided so as to be substantially layered via an inclined oscillating layering table surface through which air flows, and the layering air is guided as recycle air.
A grain separating device is known from GB-PS 1 536 905 which works in recycle-air operation and consists substantially of an oscillating table surface and a stationary box which completely encloses the table surface. The stationary box comprises a ventilator in its lower area by means of which the air is blown through the table surface from the bottom toward the top. The air flowing up from the table surface is guided back into the input of the ventilator laterally between the oscillating table surface and the walls of the stationary box. The air is guided in circular movement. This is designated as recycle-air operation. This has the great advantage that costly aspiration systems comprising corresponding filtering devices for the layering air can be dispensed with. But the fact is that these recycle-air systems have so far been successful only in very limited scope.
It has been determined in general that the devices working with recycle-air operation are either complicated with respect to construction or do not enable a satisfactory separating quality, e.g. a sufficiently high separating efficiency for the separation of stones. The reason for this consists in part in that compromises with respect to the guidance of product, particularly guiding the product in and out, and with respect to the guidance of the air must be made for recycle-air operation. In order for the oscillating table to oscillate freely, it is necessary either to provide sufficient play between the oscillating parts and stationary parts or to arrange flexible rubber bands around the entire table surface, which has a negative influence on the oscillating behavior of the table. False air normally interferes with the formation of a favorable product layering and accordingly with the successful separation of the different product components. The real problem remaining unsolved was that of the collection of dust, dirt and husk or hull parts inside the stationary box. The type of apparatus in question is predominantly that used for the separation of foreign impurities from grains and other seed materials, wherein e.g. 1-2% foreign impurities is very common. With throughput capacities of e.g. 5-20 tons per hour, there are large quantities even for the small amount of unwanted impurities. Therefore, recycle-air systems make greater demands on the plant operator because of higher expenditures for cleaning; otherwise, they are problematic with respect to hygiene. Therefore, out of the theoretical advantages there remain only great practical disadvantages. In the known recycle-air devices, dust-containing air often escaped into the environment, since the pressure conditions were not sufficiently under control.